1. Field of the Invention
The present invention relates to an image forming apparatus for controlling a replenishment quantity of a developer for a developing device based on a detection result of a toner borne-on quantity of a toner image for control, and more particularly, to control for replacing a part of control for ensuring reproducibility of an image density by adjustment of a toner charge with adjustment of a development contrast.
2. Description of the Related Art
In widely-used image forming apparatus, charged toner adheres to an electrostatic image formed on an image bearing member to develop into a toner image, the toner image is transferred to a recording material directly or through intermediation of an intermediate transfer member, and the recording material onto which the toner image has been transferred is heated and pressurized to fix an image on the recording material. As illustrated in FIG. 6B, in the image forming apparatus, the toner image is developed so that a development contrast VcontG1 being a potential difference between a potential of a developer carrying member and a potential of the electrostatic image is compensated with a total electricity amount of the charged toner.
Therefore, as a toner charge amount Q/M of the developer inside a developing device becomes higher, a toner borne-on quantity of the toner image obtained by developing a predetermined electrostatic image becomes smaller, and an image density becomes lower. On the other hand, as the toner charge amount Q/M of the developer inside the developing device becomes lower, the toner borne-on quantity of the toner image obtained by developing the predetermined electrostatic image becomes larger, and the image density becomes higher.
Accordingly, in the conventional image forming apparatus, control for maintaining the toner charge amount Q/M of the developer inside the developing device at a fixed level at all times is executed in order to enhance reproducibility of the image density (Japanese Patent Application Laid-Open No. H10-039608 and Japanese Patent Application Laid-Open No. 2005-274789).
In the image forming apparatus disclosed in Japanese Patent Application Laid-Open No. H10-039608, a fixed first development contrast is set for each image formation of a predetermined number of sheets to form a first toner image for measurement (ATR patch), and the ATR patch is detected by an optical sensor to measure the toner borne-on quantity. Then, a developer replenishment quantity for each image formation obtained by a video count method is adjusted based on output information from the optical sensor.
In the image forming apparatus disclosed in Japanese Patent Application Laid-Open No. 2005-274789, the adjustment of a development contrast at a time of image formation which uses a second toner image for measurement (Dmax patch) is performed at a frequency lower than that of the adjustment of the developer replenishment quantity which uses the first toner image for measurement. A so-called Dmax control for forming a Dmax patch having the highest density level (image ratio of 100%; solid image) and adjusting a second development contrast used for the image formation is executed. Then, in order to reduce an error in the developer replenishment quantity due to a change in temperature/humidity, the first development contrast used for developer replenishment quantity adjustment is corrected based on a relationship between the toner borne-on quantity and the development contrast obtained in the Dmax control.
In the image forming apparatus disclosed in Japanese Patent Application Laid-Open No. H10-039608 and Japanese Patent Application Laid-Open No. 2005-274789, a replenishment quantity of a replenishment developer is adjusted so that a toner charge amount of the developer is maintained at a fixed level from beginning to end. If the toner borne-on quantity of a patch toner image is small, the toner charge amount Q/M is lowered by increasing a toner replenishment quantity for the developing device and reducing a friction opportunity of the toner for a carrier, to thereby increase the toner borne-on quantity of the image. On the other hand, if the toner borne-on quantity of the patch toner image is large, the toner charge amount Q/M is increased by reducing the toner replenishment quantity for the developing device and increasing the friction opportunity of the toner for the carrier, to thereby reduce the toner borne-on quantity of the image.
If the image formation exhibiting extremely small toner consumption is continuously performed, the number of times of causing friction of the toner for the carrier increases, and hence the toner charge amount Q/M rises, while the toner borne-on quantity of the patch toner image decreases. If the toner continues to be supplied excessively in order to ensure the reproducibility of the image density by lowering the toner charge amount Q/M, a toner concentration of the developer (weight ratio of the toner to the developer) exceeds an upper limit value (for example, 11%). If the toner concentration exceeds the upper limit value, a normal operation of the developing device can no longer be continued.
On the other hand, if the image formation exhibiting extremely high toner consumption is continuously performed, the number of times of causing friction of the toner for the carrier decreases, and hence the toner charge amount Q/M decreases, while the toner borne-on quantity of the patch toner image increases. If toner replenishment continues to be stopped in order to ensure the reproducibility of the image density by raising the toner charge amount Q/M, the toner concentration of the developer drops below a lower limit value (for example, 5%). If the toner concentration drops below the lower limit value, the normal operation of the developing device can no longer be continued.
In recent years, the developing device is downsized along with a downsizing of the image forming apparatus, and the developer quantity being a denominator becomes smaller, while a maximum replenishment quantity per unit time of the replenishment developer being a numerator increases along with an increase in productivity of the image forming apparatus. Therefore, the toner charge amount Q/M of the developer inside the developing device becomes easy to fluctuate, and the toner concentration becomes easy to fluctuate along with the toner replenishment and toner replenishment restriction. Therefore, if the image formation exhibiting extremely small toner consumption is continuously performed, the toner concentration of the developer inside the developing device reaches the upper limit value in an extremely short period of time. If the image formation exhibiting extremely large toner consumption is continuously performed, the toner concentration of the developer reaches the lower limit value in an extremely short period of time.